This article is from the LAN Mail Protocols FAQ, by John Wobus firstname.lastname@example.org with numerous contributions by others.
MIME (Multipurpose Internet Mail Extensions) is a relatively new
Internet standard for the format for messages with multiple parts, and
with non-ASCII data. Any client that can import or export files can
use MIME in a clumsy way if you have a program to create and/or decode
a MIME message. Some clients have built-in features to do this.
Client-server mail protocols generally only deal with entire messages,
and can retrieve MIME messages as well as any other messages since
MIME was carefully designed to be transparent to existing mail
systems. However, IMAP4 has features to allow retrieval of individual
parts of MIME-encoded messages. The chart below lists whether a
package has MIME support. Servers for protocols that don't offer any
special MIME features are marked na for Not Applicable since they need
do nothing for users to use MIME. All IMAP4 servers can also do this,
but the chart lists whether they include explicit MIME support.